With the recent trends of the miniaturization of electronic devices and the various applications of substrates, there are increasing demands on the formation of fine wiring on thin films through a variety of printing methods, and researches on superior surface roughness and underpricing are actively going on. In particular, since there are likely a short circuit issue in the process due to surface roughness defects in multi-layer thin film structures, an adhesion strength issue due to reliability degradation, and an image quality issue in displays, researches for improving printing precision and surface roughness and lowering the prices are actively going on.
So far, silver (Ag) particles have been used for the conductive inks or pastes for printing. Compositions consisting of silver are easy to be prepared, have excellent stability and are thus stable even after printing so that they are widely applied, but since its price is changeable and high, there are increasing interests in copper (Cu)-containing inks or pastes to replace it.
Thus, Korea Patent Publication No. 2010-0118219 discloses a flexible substrate for display panel, and in particular, it describes that in order to use it as a substrate for liquid crystal display device, it is essential to develop a plastic optical film with low surface roughness and thermal expansion coefficient, and surface roughness is important because if the surface of a substrate is not flat and is very rough, it causes image quality defects in display machines.
In addition, if the surface roughness and dispersibility of metal pastes are not good, aggregates are formed, and due to this, a short circuit occurs and reliability is degraded, and since electrode thickness is not even, there is a difficulty in thinning interior electrodes. Therefore, in order to solve the above problems, Korea Patent Publication No. 2005-0104042 discloses a method for the preparation of a high dispersion metal paste for interior electrode having excellent surface roughness which is used for the preparation of high capacity multi-layer ceramic condenser (MLCC) according to recent changes in electronic device industry, but it merely prepared an evenly-dispersed paste using a 3-roll mill after dispersing metal powder in a high viscosity and separately dispersing ceramic powder in a low viscosity.
Thus, there is a demand on the development of a conductive composition (ink or paste) capable of improving surface roughness by selectively printing a copper thin film on only the desired portion of a substrate through the optimization of electrode density during sintering and the optimized dispersion of the conductive composition.